1. Field of the Invention
The present invention relates to a cycle wheel comprising a rim and a tire.
2. Description of Background and Other Information
Cycle wheels are generally comprised of a rigid portion, i.e., the rim, on which an inflatable portion, i.e., the tire, is mounted. Two types of tires are commonly used for bicycle wheels; tubular tires and wire bead tires, the latter commonly referred to as pneumatic tires, sometimes as clincher tires, or, simply, as tires. The invention encompasses both of these tire types.
The amount of power a human being can generate is limited; therefore, the forward speed of a cyclist is limited. This speed results from a balance between the power developed by the cyclist and the forces opposing the advance, i.e., the forward motion. The opposing forces include, in particular, rolling resistance, which is independent of the speed and the aerodynamic resistance, and the drag, which is a function of speed squared.
The theory of aerodynamics teaches that at low Reynolds numbers, the more laminar the air flow around the moving object, the lower the drag. For this reason, configurations that generate separation of the air-flow are sought to be avoided, or, in any case, configurations are sought that limit air-flow separation as much as possible.
In the case of a cycle wheel, in particular a spoked wheel, because of its annular characteristic and because the direction of its advance through air is within its plane of symmetry, the shape of the tire as well as that of the rim determine the aerodynamic drag on the wheel. Indeed, depending upon the direction of advance of the wheel with respect to the front portion of the wheel, the tire constitutes the leading edge for penetration through the air, and the trailing edge is at the rim. Conversely, with respect to the rear portion of the wheel, the leading edge is at the rim and the trailing edge is at the tire.
Several manufacturers have proposed models of wheels which are claimed to reduce aerodynamic resistance. For example, the document U.S. Pat. No. 5,061,013 discloses a cycle wheel in which the ratio of the maximum width of the rim to the width of the tire is greater than 1. As another example, the document FR 2 555 106 discloses the positioning of a banding strip on a tire in order to center the tire on the rim and to reduce the aerodynamic penetration coefficient.
All such attempts have provided no satisfactory solution to the problem of reducing aerodynamic resistance. As will be seen hereinafter, the aerodynamic drag is often decreased for either the front or rear portion of the wheel, but not for both.
The rim disclosed in the document U.S. Pat. No. 5,061,013 does not comply with the standard of the ETRTO (European Tyre and Rim Technical Organization). In particular, this standard recommends that the recess of the rim not exceed 4.5 millimeters (mm) for a tubular rim. In the rim of this document, the rim recess is very substantial, which necessarily has consequences either on the mounting of the tire or on maintaining the tire on the rim. Indeed, if the outer diameter is that of a standardized rim, the tire can be mounted easily; however, due to the substantial size of the recess, the tire, once mounted on the rim band, will be loose. If the diameter of the rim band corresponds to the ETRTO recommendations, it will certainly be very difficult, during assembly, to pass the tire over the beads of the rim. In addition, the wheel obtained is not very comfortable insofar as, due to the substantial rim recess, the rim flange travel is reduced. A reduced rim flange travel multiplies the risks of puncture by crushing the tire and pinching the tube. To decrease this risk, the designer of the wheel disclosed in U.S. Pat. No. 5,061,013 chose to have thick and rounded upper rim edges. The thickness of the upper edges of the rim results in breaking the continuity of the outer surface of the wheel in the area of the tire/rim interface. This break of continuity is a source of disturbance in the air flow and therefore increases aerodynamic resistance. This is particularly the case for the rear portion of the wheel, i.e., whereby the leading edge is the rim and the tire is the trailing edge. Indeed, the width of the rim is much more substantial than that of the tire, which increases the frontal surface.
The document FR 2 555 106 proposes a banding strip to improve the connection of the edges of the rim to the tire. In this construction, the narrowing of the rim with respect to the tire has the consequence that, with respect to the front portion of the wheel, the air-flow separation limit is between the zone of the greatest width of the tire and the rim, causing a disturbance in the flow along the side walls of the rim. The separation of the air-flow generates a strong depression in the vicinity of the maximum width of the tire, which increases the aerodynamic drag. In the zone of the banding strip, the air-flow is already separated, so that this zone has no effect on the decrease in the drag. Indeed, the wheel disclosed in this document has almost the same aerodynamic drag, with or without a banding strip. In addition, as described in this document, the banding strip comprises edge beads to provide consistency with the elastic or plastic material that forms the connection profile. Such a construction is relatively heavy because, in the case of a tube, it involves providing the tube with a reinforcing element that is typical of wire bead tires, and in the case of a wire bead tire, it involves doubling this reinforcing structure. Consequently, even if there were improvement to the aerodynamics, such would be to the detriment of the weight of the wheel.